By Anthony J. Brown, MD

NEW YORK (Reuters Health) – The increase in autism prevalence that many studies have reported in recent years may be attributable, at least in part, to a drop in the age at diagnosis over time, the results of a Danish study suggest.

“Our study is the first study to quantify how decreasing age of diagnosis inflates the observed prevalence of autism,” lead author Dr. Erik T. Parner, from the University of Aarhus, told Reuters Health. “We were surprised that the impact of shifts in age of diagnosis on the observed autism prevalence was so great.”

The study, which is reported in the December issue of the Archives of Pediatrics and Adolescent Medicine, included all 407,458 children who were born in Denmark from 1994 to 1999.

In children who were diagnosed with autism spectrum disorder before 9 years of age, the average age at diagnosis fell in more recent birth cohorts. For the 1994-1995, 1996-1997, and 1998-1999 cohorts, the average ages at diagnosis were 5.9, 5.8, and 5.3 years, respectively.

Compared with the 1994-1995 birth cohort, the 1996-1997 cohort was 20% more likely to be diagnosed with autism spectrum disorder at 3, and 10% more likely to be diagnosed at 11 years of age. Similarly, relative to the 1996-1997 cohort, the 1998-1999 cohort was 69% and 23% more likely to be diagnosed at 3 and 11 years of age, respectively.

“The important conclusion to be made is that the shifts in age at diagnosis — especially the earlier diagnosis at younger ages — artificially inflated the differences in the observed prevalence of autism in young children in the more recent cohorts compared with the cohort of the oldest children,” the investigators explain.

“The increasing prevalence of autism needs to be understood in greater detail,” Dr. Parner said. “We are currently studying the impact of changes in diagnostic criteria and changes in perinatal and parental psychiatric history risk factors over time.”

Reference:
Arch Pediatr Adolesc Med 2008;162:1150-1156.